1. Field of the Invention
One aspect of the present invention is related to methods for enhancing NH3 adsorption capacity of selective catalytic reduction catalysts.
2. Background Art
Environmental concerns and governmental regulations have been a continuing impetus for improvements in pollution control from automotive vehicles. The treatment or removal of noxious combustion by-products from the vehicle exhaust stream is a major focus of such efforts. Typically, these combustion by-products include incomplete combustion by-products such as carbon monoxide and hydrocarbons. Moreover, the exhaust of these vehicles may include various nitrogen oxides (NOx) and sulfur oxides. It is desirable and, in certain instances, mandated that certain of these compounds be reduced during vehicle operation.
Currently, the exhaust systems of automotive vehicles include one or more catalyst systems to affect the control of such by-products. One known system utilizes a selective catalytic reduction (SCR) process to control emissions of potentially harmful by-products, e.g. to reduce the amount of NOx exiting the vehicle exhaust. These systems have been implemented in lean burn engine vehicles, such as diesel engine vehicles. The system utilizing the SCR process, i.e. the SCR system, includes a substrate upon which a SCR catalyst composition is applied (via a wash coating process, for example). The SCR catalyst composition includes an SCR catalyst, which can be a vanadium-based catalyst or zeolite-containing catalyst. According to one SCR process, a gaseous or liquid reductant is added to a flue gas stream and is adsorbed onto a SCR catalyst. NH3 can be used as the reductant, and can be delivered in gaseous form or via urea injection with subsequent decomposition to NH3.
The reduction of NOx with NH3 over SCR catalysts can be significantly influenced by the amount of NH3 adsorbed on the catalysts. An increase in NH3 adsorption capacity can increase the catalytic rate of reduction. In many applications, the SCR reaction occurs at temperatures above 150° C., and therefore, it may be desirable in such applications to use have SCR catalysts that have a substantial adsorption capacity of NH3 at temperatures that exceed 150° C. In certain applications, it is desirable to have SCR catalysts possessing a relatively high capacity of NH3 adsorption and relatively strong bonding to NH3.
One common drawback of certain known SCR systems is NH3 slip, which occurs when exhaust gas temperatures are too cold for the SCR reaction to occur and/or too much reductant is fed into the exhaust gas stream for the amount of NOx in the stream. A variety of strategies have been developed to deal with NH3 slip, including the fitting of extra catalysts after the SCR catalyst.
In light of the foregoing, methods and formulations relating to SCR for addressing one or more of the disadvantages and/or providing one or more of the benefits identified above is needed.